A complex metal oxide used as an adsorbent for removing fluorine from water is prepared from the transition metal chosen from Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, aluminum and RE metal chosen from Ce and La in the mole ratio of (0.5-1.5): (2-6): (0.5-1.5) through dissolving them in water to become aqueous solution, dropping alkali solution to make its pH=9.0-9.5, laying aside, centrifugal washing and separating by deionized water, drying, and grinding.